Chap. II.

COMPOSITION AND USE OF MINERALS.

667

and that put into pot No. 2. is melted, and is treated in the same way as the lead in potNo. 1. The lead taken away is then put into pot No. 3., and melted and treated in thesame manner, when it is found to contain so little silver that it will not defray the expenseof melting it a fourth time.

In some mines the lead is richer, as at Greenside, where there are five pots in theseparating room ; the lead of this mine contains from 12 to 14 ounces of silver to the ton,and there is sufficient silver, after it has been melted and separated the third and fourtlitime, to cover the expense, and yield a profit in melting it again.

Of the reducing Furnace -Under the new system, before the remaining lead and silver

is subjected to refining, comparatively little litharge is made, although there is more thancan he sold at a remunerating price, either to the glass-makers or the colour-makers;much, therefore, is reduced to metal in the reverberatory furnace, at the bottom of whichis placed a layer of coals, and the litharge, mixed with small coal, is put in and exposedto the flames. During the combustion the small coal abstracts the oxygen from thelitharge, and pure lead is the result, which is cast into pigs of 12 stones each, and is in amalleable state.

Reducing the Slag _The slag is put into a furnace, mixed with coke and heated by

fuel beneath : the oxygen of the slag enters into combination with the fuel, and thelead is separated and cast into pigs. This is less valuable than the other, and is easilybroken.

Cast Lead is manufactured by melting the pig-lead in a large iron vessel, and then ladlingIt out on a table 18 or 20 feet in length, which has been previously covered with finepressed and beaten sand, brought to a level and smooth surface by passing a strikeover it. The table has a rising edge all around it, on the top of which is a movablestrike, which determines the thickness to be given to the sheet; this strike, when the metalis in a liquid state, sweeping before it the superabundant lead. When a very thin sheet isrequired to be cast, a linen cloth is stretched over another of wool, on which is pouredthe lead, care being taken that the heat is not sufficient to set fire to the paper, and it isrequisite, as the lead cools rapidly, to be very adroit in passing the strike over it.

Milled Lead is first cast in sheets, and then passed under rollers, placed at such a distanceapart as is required for its thickness, the space between each pair of rollers it passesthrough diminishing gradually : the weight in pounds of a superficial foot is

lbs.

When of a sixteenth of an inch in thickness

a twelftha tenth -an eightha sixth -a fifth

10

12

a quartera third -a half

The weight of a cubic foot of lead is 709.} pounds, and Smeaton found that a bar 12inches long, 1 inch square, and weighing 4 - 94 pounds, was expanded by 1 degree of heatLead melts at 612 degrees, and will bear a weight of 1300 pounds on each square inch,without altering its form materially. Compared with cast-iron its strength is *096; itsextensibility 2-5 times, and its stiffness ’0385 times.

Casting of Leaden Pipes is comparatively a recent invention; for those used in water-works were commonly made of sheet lead wrapped round a wooden or iron core, and where

/iMPCAES ADRIAN! AVC'

Fig •"»84.

Tim FOUND AT LYONS.